1. The Field of the Invention
The present invention relates to forced-air heaters that are relatively compact, yet provide high outputs of heated air. More particularly, the present invention relates to a forced-air heater that is self-regulating and automatically adjustable between different heat output settings in order to quickly heat an environment to a selected temperature and thereafter efficiently maintain the environment at the selected temperature.
2. The Relevant Technology
Forced-air heaters are also known as space heaters and have traditionally been used to heat large open spaces that require high volumes of heated air in order to maintain a comfortable environment. In order to efficiently use forced-air heaters to heat smaller spaces, recent steps have been taken to reduce the size of the forced-air heaters and to make them portable. An early portable forced-air heater is described in U.S. Pat. No. 3,494,599, issued to Joseph J. Stupak, Jr., et al.
Traditionally, forced-air heaters are contained within a single housing. An air passageway extends through the housing from an air inlet opening to an air outlet opening. A blower draws air in through the air inlet opening and forces it into a combustion chamber within the air passageway. A burner at or near the entrance to the combustion chamber supplies a mixture of fuel and air into the combustion chamber, where the fuel and air mixture is ignited. The resulting combustion heats the air as it passes through the combustion chamber to the air outlet opening.
The fuel, typically propane or natural gas, is supplied to the burner by a venturi tube. The venturi tube typically comprises a fuel-air mixture pipe into which fuel is released from a fuel supply under pressure. Outside air is also allowed to enter the fuel-air mixture pipe and mix with the fuel. A venturi effect caused by the air passing through the combustion chamber draws the mixture of fuel and air into the combustion chamber, where the fuel and air mixture is ignited.
A platelike flame spreader is frequently employed in such forced-air heaters and is normally located between the blower and the combustion chamber to create a low pressure region around the edges of the burner. The low pressure region tends to spread out the area in which combustion takes place, which causes the fuel and air mixture to more fully combust. Turbulence is also created by the indirect path that the air must take to enter the combustion chamber due to the flame spreader. The turbulence causes the blower-driven air to better mix with the fuel-air mixture from the fuel and air mixture pipe, also helping the fuel to combust more thoroughly. Thorough combustion efficiently utilizes the fuel and also reduces carbon monoxide emissions.
One drawback to such designs is that the conventional forced-air heaters have been capable of heating air at only a single, constant heat output setting. One reason for this limitation is that the flame spreaders in conventional designs are sized to draw only a single, specified volume of fuel and air from the gas-air mixture pipe into the combustion chamber and spread that volume of the fuel and air mixture over a specified distance. Not spreading the fuel and air mixture far enough results in insufficient combustion and possibly, high carbon monoxide emission. Spreading the fuel and air mixture too far results in inconsistent combustion and excessive deposits within the combustion chamber.
Attempts have been made to increase the ranges of fuel and air mixture volumes that will burn cleanly and without buildups in a given forced-air heater. These attempts have included strategic placement of holes, fins and other air routing mechanisms in and on the flame spreader to better disperse and mix the fuel and air mixture with the air forced into the combustion chamber by the blower. Nevertheless, to date these attempts have not realized a satisfactory performance at more than a single heat output setting. The single heat output setting thus remains a limitation of conventional forced-air heaters.
In regulating the heat output of the conventional forced-air heaters provided with only a single heat output setting, the flow of fuel into the combustion chamber is alternately enabled and disabled. Thus, the typical operating cycle of these forced-air heaters begins operation at the standard heat output setting in which a predetermined fuel pressure and volume is passed into the combustion chamber at a predetermined blower speed until the environment being heated attains a selected temperature. Once the selected temperature is attained, the flow of fuel is disabled until the temperature of the environment being heated drops below the selected temperature to a predetermined extent. At that point, the flow of fuel is again enabled. This enablement/disablement sequence is repeated continuously during the operating cycle, and the blower may or may not be disabled concurrently with the disablement of the flow of fuel. This repeated enablement/disablement sequence is an inefficient use of fuel, and leaves much to be desired in maintaining a constant temperature of the environment being heated.
In order to lengthen the time intervals occurring between the enabling and the disabling of the flow of fuel, the standard heat output settings of conventional forced-air heaters are normally set at relatively low heat output settings. Consequently, when powering up a conventional forced-air heater after a period of non-use in which the temperature of an environment has dropped substantially below a desired temperature, the conventional forced-air heater takes an undesirably long period of time to heat the environment to the desired temperature.